Add create-2d-physics skill

[MelvMay-Unity]

Adds create-2d-physics, which creates, fixes and explains Unity 2D physics in both systems: 2D Physics Core (the Unity.U2D.Physics API and the com.unity.2d.physics components) and the older Rigidbody 2D system.

Audit results

Check	Result
create-or-check-enduser-skill checklist	53 of 58 pass
Its checker script	20 of 20 pass
Docs audit against 6000.7	No errors, all links resolve
Trigger evals (agent-skill-eval)	15 of 15
Functional evals, 10 cases × 3 runs	0.947

The evals ran locally, not in CI.

Checklist items not followed, on purpose

Check	Reason
P1, and the Debug.Log part of P2	A compile check is the verification. Play-mode logging made simple tasks slow.
P4, D2	Docs are read only when a task needs them. Mandatory reading before every step made simple tasks slow.
M9	The agent-skill-eval lint warns that required_editor_version is an unknown key.

Documentation gaps the audit found

• The 6000.7 minimum for the com.unity.2d.physics components, and that the script API needs no package
• That both 2D physics modules are on by default
• An ownership concept page
• A task page for queries
• Drawing from script, on the debug drawing page
• The apply call needed after changing a definition or geometry from script
• When to use Primitive rather than a dedicated Area component
• The default body type for a script-created body